1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor, and an electric power steering apparatus including such a magnetostrictive torque sensor.
2. Description of the Related Art
Magnetostrictive torque sensors are known in the art. The sensors include detection coils for electrically detecting magnetostriction which magnetostrictive rotational shafts, when subjected to torques, cause. Such magnetostrictive torque sensors are disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI 6-221940 entitled xe2x80x9cMAGNETOSTRICTIVE TORQUE SENSORxe2x80x9d and Japanese Patent Laid-Open Publication No. HEI 9-166505 entitled xe2x80x9cMETHOD FOR MANUFACTURING MAGNETOSTRICTIVE TORQUE SENSORxe2x80x9d.
A magnatostrictive torque sensor disclosed in the publication No. HEI 6-221940 includes a substantially 8-shaped excitation coil and a substantially 8-shaped detection coil disposed on the excitation coil at right angles thereto. The sensor lies in parallel to an axis of a magnetostrictive torque transmission shaft. When a torque is applied to the shaft, the shaft causes magnetostriction in correspondence to the torque. The sensor electrically detects the magnetostriction.
A magnetostrictive torque sensor disclosed in the publication No. HEI 9-166505 includes first and second magnetic anisotropy portions formed on a rotational shaft. The first and second magnetic anisotropy portions are aligned along a longitudinal direction of the shaft. Each of the first and second portions comprises a plurality of magnetostrictive narrow films. The films can be formed using vapor plating. The films of each of the first and second magnetic anisotropy portions are disposed around a circumference of the rotational shaft. The films of the first magnetic anisotropy portion are spaced at given intervals. Similarly, the films of the second magnetic anisotropy portion are spaced at given intervals. Each film of the first and second magnetic anisotropy portions is inclined relative to the longitudinal direction of the shaft. More specifically, the films of one of the first and second magnetic anisotropy portions are inclined in a clockwise direction while the films of the other are inclined in a counterclockwise direction. The sensor also includes detection coils. The detection coils are designed to electrically detect magnetostriction which the first and second magnetic anisotropy portions, when subjected to a torque, cause.
However, the torque transmission shaft used with the torque sensor disclosed in the publication HEI 6-221940 should be made from material capable of causing magnetostriction. This provides limited freedom to select material from which the torque transmission shaft is made. Also, the rotational shaft is subjected to heat treatment to thereby provide decreased sensitivity of the sensor. Such a sensor of decreased sensitivity should be provided with an amplifier which provides a high gain even in the presence of noise. It is not desirable that the sensor of decreased sensitivity be used with parts of large strength such as a steering shaft.
The sensor disclosed in the publication HEI 9-166505 includes the films disposed on the rotational shaft, as described above. These films should be subjected to masking with increased precision such that they are spaced at the given intervals. It is therefore difficult to produce a number of the rotational shafts having the films formed thereon. Further, the rotational shaft should be made from non-magnetic material, otherwise the sensor would provide an output of small level. Furthermore, since the non-magnetic is relatively expensive, the rotational shaft is costly to make. This results in increased cost of the torque sensor.
It is an object of the present invention to provide a less costly magnetostrictive torque sensor which has increased mechanical strength and improved durability and which is designed to provide unlimited freedom to select material from which a rotational shaft is made. A further object of the present invention is to provide an electric power steering apparatus including such a magnetostrictive torque sensor and rotational shaft.
According to a first aspect of the present invention, there is provided a magnetostrictive torque sensor comprising: a magnetostrictive film having a predetermined width, the magnetostrictive film being configured to be provided along a circumference of a rotational shaft, the magnetostrictive film having a magnetic path passing therethrough; a substantially U-shaped excitation core disposed in opposed relation to the magnetostrictive film, the excitation core extending through an excitation coil; a substantially U-shaped detection core disposed in opposed relation to the magnetostrictive film, the detection core extending through a detection coil, the detection core being disposed on the excitation core at right angles thereto; a magnetic circuit provided by the magnetic path, the excitation coil, and the detection coil; and a bridge circuit provided by the excitation coil and the detection coil, wherein the magnetostrictive film causes magnetostriction in correspondence to a torque applied to the rotational shaft, and the magnetostriction is electrically detected by the bridge circuit.
The torque sensor includes the magnetostrictive film provided along the circumference of the rotational shaft. That is, the film can be readily provided on the rotational shaft. Thus, the torque sensor is made simple in construction. Although the film is made from expensive material, the film is of small thickness, preferably 1 mm or less in thickness, and hence the material required to form the film can be made small in amount. It thus becomes possible to reduce the cost of the film. Further, since the film is provided along the circumference of the rotational shaft, it can cause magnetostriction to a greater extent. Accordingly, the torque sensor provides improved sensitivity thereof. The torque sensor thus arranged need not include the amplifier as used in the sensor disclosed in the publication HEI 6-221940. Still further, since the film provides skin effect, it is less likely that the film, when causing magnetostriction, is affected by material from which the rotational shaft is made. Moreover, the sensor is of improved sensitivity, and therefore the rotational shaft can be subjected to heat treatment. The thus heat-treated rotational shaft has increased mechanical strength and improved durability. Such a rotational shaft can be used as a steering shaft. The substantially U-shaped excitation core is disposed on the substantially U-shaped detection core at right angles thereto. The cores are disposed in opposed relation to the film. The excitation coil and the detection coil extending through the excitation core and the detection coil, respectively, cooperate with each other to provide the bridge circuit for electrically detecting magnetostriction which the film causes in correspondence to a torque applied to the rotational shaft. The bridge circuit can detect the magnetostriction with increased precision. The excitation core and the detection core have increased strength and improved durability, and thus they can be used with a vehicle steering shaft or the like. Also, since the torque sensor is simple in construction, it becomes possible to manufacture the sensor with ease.
The bridge circuit detects the magnetostriction caused by the film provided on the circumference of the rotational shaft. In other words, the material from which the rotational shaft is made does not affect the detection of the torque applied to the rotational shaft. Therefore, the rotational shaft can be freely designed in such a manner as to provide a reduced cost of the magnetostrictive torque sensor.
According to a second aspect of the present invention, there is provided a magnetostrictive torque sensor comprising: a substantially 8-shaped excitation coil; a substantially 8-shaped detection coil disposed on the excitation coil at right angles thereto; and a magnetostrictive film having a predetermined width, the magnetostrictive film being configured to be provided along a circumference of a rotational shaft, the magnetostrictive film being disposed in opposed relation to the excitation coil and the detection coil, wherein the magnetostrictive film causes magnetostriction in correspondence to a torque applied to the rotational shaft, and the magnetostriction is electrically detected by the detection coil.
The torque sensor includes the magnetostrictive film provided along the circumference of the rotational shaft. That is, the film can be readily provided on the rotational shaft. Thus, the torque sensor is made simple in construction. Although the film is made from expensive material, the film is of small thickness, preferably 1 mm or less in thickness, and hence the material required to form the film can be made small in amount. Thus, it becomes possible to reduce the cost of the film. Further, since the film is provided along the circumference of the rotational shaft, it causes magnetostriction to a greater extent. Accordingly, the torque sensor provides improved sensitivity thereof. The torque sensor thus arranged need not include the amplifier as used in the sensor disclosed in the publication HEI 6-221940. Still further, since the film provides skin effect, it is less likely that the film, when causing magnetostriction, is affected by material from which the rotational shaft is made. Moreover, the sensor is of improved sensitivity, and therefore the rotational shaft can be subjected to heat treatment. The thus heat-treated rotational shaft has increased mechanical strength and improved durability. Such a rotational shaft can be used as a steering shaft. The substantially 8-shaped excitation coil is disposed on the substantially 8-shaped detection coil at right angles thereto. These coils are disposed in opposed relation to the film. This arrangement leads to improved durability of the coils. Accordingly, the excitation coil and the detection coil thus arranged can be used with the steering shaft or the like. Also, since the torque sensor is simple in construction, it is easy to manufacture the sensor.
The excitation coil detects the magnetostriction caused by the film provided on the circumference of the rotational shaft. In other words, the material from which the rotational shaft is made does not affect the detection of the torque applied to the rotational shaft. Therefore, the rotational shaft can be freely designed in such a manner as to provide a reduced cost of the magnetostrictive torque sensor.
According to a third aspect of the present invention, there is provided an electric power steering apparatus comprising: a rotational shaft including a steering wheel; a torque sensor for detecting a steering torque transmitted to the rotational shaft by means of the steering wheel; an electric motor for causing an assisting torque on the basis of the steering torque detected by the torque sensor; and a transmission means for transmitting the assisting torque to the rotational shaft; the torque sensor including: a magnetostrictive film having a predetermined width, the magnetostrictive film being configured to be provided along a circumference of a rotational shaft, the magnetostrictive film having a magnetic path passing therethrough; a substantially U-shaped excitation core disposed in opposed relation to the magnetostrictive film, the excitation core extending through an excitation coil; a substantially U-shaped detection core disposed in opposed relation to the magnetostrictive film, the detection core extending through a detection coil, the detection core being disposed on the excitation core at right angles thereto; a magnetic circuit provided by the magnetic path, the excitation coil, and the detection coil; and a bridge circuit provided by the excitation coil and the detection coil, wherein the magnetostrictive film causes magnetostriction in correspondence to the steering torque applied to the rotational shaft, and the magnetostriction is electrically detected by the bridge circuit.
A rotational shaft used in a conventional electric power steering apparatus is horizontally divided into two shafts. These two shafts are connected to each other through a torsion bar. However, the rotational shaft used in the electric power steering apparatus of the present invention need not include such a torsion bar. Accordingly, the rotational shaft becomes simple in construction.
The rotational shaft is provided with the magnetostrictive film rather than with the torsion bar. The film is provided along the circumference of the shaft. The film causes magnetostriction even in a case where a portion of the shaft having the film provided thereon twists a very small amount in response to the application of the steering torque to the rotational shaft. The magnetostriction is detected by the bridge circuit.
According to a fourth aspect of the present invention, there is provided an electric power steering apparatus comprising: a rotational shaft including a steering wheel; a torque sensor for detecting a steering torque transmitted to the rotational shaft by means of the steering wheel; an electric motor for causing an assisting torque on the basis of the steering torque detected by the torque sensor; and a transmission means for transmitting the assisting torque to the rotational shaft; the torque sensor including: a substantially 8-shaped excitation coil; a substantially 8-shaped detection coil disposed on the excitation coil at right angles thereto; and a magnetostrictive film having a predetermined width, the magnetostrictive film being configured to be provided along a circumference of a rotational shaft, the magnetostrictive film being disposed in opposed relation to the excitation coil and the detection coil; wherein the magnetostrictive film causes magnetostriction in correspondence to the steering torque applied to the rotational shaft, and the magnetostriction is electrically detected by the detection coil.
A rotational shaft used in a conventional electric steering apparatus is horizontally divided into two shafts. These two shafts are connected to each other through a torsion bar. However, the rotational shaft used in the electric power steering apparatus of the present invention need not include such a torsion bar. Accordingly, the rotational shaft becomes simple in construction.
The rotational shaft is provided with the magnetostrictive film rather than with the torsion bar. The film is provided along the circumference of the shaft. The film causes magnetostriction even in a case where a portion of the shaft having the film provided thereon twists a very small amount in response to the application of the steering torque to the rotational shaft. The magnetostriction is detected by the detection coil.